1. Field of the Invention
The present invention relates to a method of calibrating zero offset of a pressure sensor. More particularly, the present invention relates to a method of forming a stress adjusting thin film on at least a surface of a diaphragm of a pressure sensor to calibrate a zero offset of the pressure sensor.
2. Description of the Prior Art
A pressure sensor is one of the familiar components of MEMS products, and a piezoresistive pressure sensor is the most common pressure sensor at present. Please refer to FIG. 1. FIG. 1 is a theoretical structure of a prior art piezoresistive pressure sensor 10. The prior art piezoresistive pressure sensor 10 has a Wheatstone bridge arrangement, and operates on the theory that the resistance of a piezoresistor changes with the pressure. As shown in FIG. 1, the Wheatstone bridge contains four variable resistors R1, R2, R3 and R4. The input voltage (Vin) is connected to two points, one point being between resistors R1 and R2 and the other point being between resistors R3 and R4. The output voltage (Vout) is connected to two points, one point being between resistors R1 and R3 and the other point being between resistors R2 and R4. The relationship between Vin and Vout is given by:Vout=Vin*ΔR/R 
Where R is the initial resistance of resistors R1, R2, R3 and R4, and ΔR is the change of resistance of resistors R1, R2, R3 and R4. The variable resistors of the piezoresistive pressure sensor 10 are piezoresistors. The resistances of resistors R1 and R3 decrease with pressure as the resistances of resistors R2 and R4 increase. With a constant input voltage, a change of output voltage is proportional to a change of resistance. Thus, the piezoresistive pressure sensor 10 represents a change of pressure with the output voltage.
Please refer to FIG. 2, and also refer to FIG. 1. FIG. 2 is a schematic diagram of the prior art piezoresistive pressure sensor 10. As shown in FIG. 2, the prior art piezoresistive pressure sensor 10 includes a diaphragm 12, a pressure sensing component 14 disposed in the diaphragm 12, and a stand 16 positioned under the diaphragm 12 to support the diaphragm 12. The diaphragm 12 is generally made from a semiconductor material, such as silicon. The circuit of pressure sensing component 14 is shown in FIG. 1, wherein the process of forming the circuit includes forming the piezoresistors in the diaphragm 12 by doping and diffusing techniques, then forming conducting wires by depositing and etching techniques, etc. The material of the stand 16 may be semiconductor material or glass material. As above, the pressure on the surface of the piezoresistive pressure sensor 10 deforms the variable resistors R1, R2, R3, and R4, and the deformation changes the output voltage, so that a change of pressure can be measured.
Factors such as process accuracy, temperature, stress, and difference of thermal expansion coefficient of materials can affect the offset of a Wheatstone bridge, and therefore, a significant zero offset may exist. As soon as the zero offset increases excessively, the difficulty of circuit design increases, the performance of the piezoresistive pressure sensor 10 decreases, and the cost of calibration increases. Please refer to FIG. 3. FIG. 3 is a schematic diagram of the prior art piezoresistive pressure sensor 10 with a large zero offset. As shown in FIG. 3, the factors such as stress and difference of thermal expansion coefficient slightly deform the piezoresistive pressure sensor 10, changing the resistance of the variable resistors R1, R2, R3, and R4 in the initial condition. As a result, the zero offset of the piezoresistive pressure sensor 10 becomes too large in the initial condition. To solve this problem, the prior art changes the resistance of the variable resistors R1, R2, R3, and R4 by laser trimming to calibrate the zero offset after the piezoresistive pressure sensor 10 is made.
Although laser trimming can change the resistance of the variable resistors effectively, laser trimming can only be used for higher-end products because of its high cost. In addition, a limitation of laser trimming is that the piezoresistive pressure sensors must be calibrated one by one, not in a batch process. In consideration of this limitation, a method of calibrating a zero offset of piezoresistive pressure sensors by a stress-adjusting thin film is proposed. This method possesses the characteristics of low cost and conduciveness to batch production, which can solve the limitations of high cost and inefficiency of the prior art.